Growth and differentiating factors that are found in the eye guide the development of individual ocular tissues. Their interactions with other eye tissues to form a functional visual system. We have identified a new and unique differentiating factor secreted from fetal human pigment epithelial cells, called pigment-epithelial differentiating factor, that is neurotropic to cultured human retinoblastoma cells and may affect neural retinal development in vivo. Neurotropic activity is also present in adult bovine interphotoreceptor matrix. Ocular tissues synthesize insulin-like growth factor (IGF-1) binding proteins, thus may self-regulate activity of this important growth factor in the vitreous and surrounding tissues.